1. Field of the Invention
The present invention generally relates to a method of delivering bioactive substances, particularly nucleic acids, into cells. This invention describes a series of novel biodegradable polyphosphoesters as transport agents. Preferred polymers of the invention self assemble in aqueous buffer at room temperature into micelles. Negative charged molecules, e.g. plasmid DNA form complex with these vesicles. Drugs or molecules that can be delivered using these polymeric systems range from DNA plasmids, RNAs, proteins to small molecular weight drugs. Polymers of the invention can also be used to aid the virus particle mediated gene transfer.
2. Background
Effective delivery of nucleic acid to cells or tissue with high levels of expression are continued goals of gene transfer technology. As a consequence of the general inability to achieve those goals to date, however, clinical use of gene transfer methods has been limited.
Ideal gene delivery vehicles should be bioabsorable, non-toxic, non-immunogenic, stable during storage and after administration, able to access target cells, and suitable for efficient gene expression. As many studies demonstrate, the limitations of viral vectors make synthetic vectors an attractive alternative.
Cationic liposome (Lipoplex) and cationic polymers are among the two major types of non-viral gene delivery vectors. Toxicity data on cationic polymers suggests that many polymers used for transfections are most effective at concentrations that are just subtoxic. illustrative polymers include polyamino acids (e.g. poly-L-lysine, poly-L-ornithine), polyamidoamine dendrimers, chitosan, polyethylenimine, poly((2-dimethylamino)ethyl methacrylate). The entry of the complexes may be mediated by the membrane destabilizing effects of cationic polymers. Several observations have suggested that liposomal systems are relatively unstable after the administration. Significant toxicity has been shown to be associated to liposomal vectors, especially the fusogenic phospholipid (neutral lipid), include the down regulation of PKC dependent immunomodulator synthesis, macrophage toxicity, neurotoxicity, acute pulmonary inflammation, etc. Moreover, both vectors are far less efficient comparing with the viral counter-part. Searching for a safe and efficient gene carrier will still remain a major challenge in the field of non-viral gene delivery.
Biocompatible polymeric materials have been used extensively in therapeutic drug delivery and medical implant device applications. Sometimes, it is also desirable for such polymers to be, not only biocompatible, but also biodegradable to obviate the need for removing the polymer once its therapeutic value has been exhausted.
Conventional methods of drug delivery, such as frequent periodic dosing, are not ideal in many cases. For example, with highly toxic drugs, frequent conventional dosing can result in high initial drug levels at the time of dosing, often at near-toxic levels, followed by low drug levels between doses that can be below the level of their therapeutic value. However, with controlled drug delivery, drug levels can be more nearly maintained at therapeutic, but non-toxic, levels by controlled release in a predictable manner over a longer term.
If a biodegradable medical device is intended for use as a drug delivery or other controlled-release system, using a polymeric carrier is one effective means to deliver the therapeutic agent locally and in a controlled fashion, see Langer et al., Rev. Macro. Chem. Phys., C23(1), 61 (1983). As a result, less total drug is required, and toxic side effects can be minimized. Polymers have been used as carriers of therapeutic agents to effect a localized and sustained release. See Chien et al., Novel Drug Delivery Systems (1982). Such delivery systems offer the potential of enhanced therapeutic efficacy and reduced overall toxicity.